1. Field of the Invention
This invention relates to a transparent electromagnetic radiation shield material for placement in front of a display device or the like to block electromagnetic radiation, and to a method of producing the material. The transparent electromagnetic radiation shield material is particularly suitable for a large plasma display.
2. Description of the Background Art
An electromagnetic radiation shield material for placement in front of a display device or the like is required to have not only excellent electromagnetic radiation shielding capability but also excellent transparency (optical transmittance), good clarity (degree of coating blackness etc.), wide viewing angle and the like. Japanese Patent Application Laid-Open Nos. 9-298384 (""384) and 10-41682 (""682) teach electromagnetic radiation shield materials meeting these requirements to some extent.
Specifically, ""384 teaches a method wherein xe2x80x9ca step of providing a black dyed layer on a transparent base material, a step of providing a metallic layer on the black dyed layer, a step of providing a patterned resist layer on the metallic layer and a step of removing portions of the metallic layer not covered by the resist layer by etching with an etching solution are conducted in succession, portions of the black dyed layer not covered by the patterned metallic layer being decolored by the etching solution in the etching step.xe2x80x9d
On the other hand, ""682 teaches a method wherein xe2x80x9clines constituting a geometric pattern drawn on the surface of a transparent plastic base material with an electrically conductive material have a line width of 40 xcexcm or less, a line interval of 200 xcexcm or greater and a line thickness of 40 xcexcm or less, part or the whole surface of the base material including the geometric pattern is coated with an adhesive, the difference between the refractive indices of the adhesive coating the geometric pattern and the transparent plastic base material is made 0.14 or less or, if the transparent plastic base material is laminated on an electrically conductive material via an intervening adhesive layer, the difference between the refractive indices of the adhesive layer and adhesive coating the geometric pattern is made 0.14 or less, to obtain an adhesive film having electromagnetic radiation shielding property and transparency, and the result is used as a display electromagnetic radiation shielding component.xe2x80x9d
When a black dye is used in a black resin layer as in ""384, however, it is generally hard to obtain a sufficient degree of coating blackness and good clarity. The dye content and/or the resin layer thickness therefore has to be increased.
In addition, when the etching solution for the metallic layer is used to decolor and extract the black dye, the metallic layer comes to be over-etched owing to the long time needed for the decoloration.
As the electromagnetic radiation shield material is ordinarily used after peeling off the resist layer, moreover, the side facing the display exhibits metallic glare and reflects the display, making it hard to view. Another problem is that the metallic layer readily oxidizes in air. Its conductivity (electromagnetic radiation shielding performance) therefore tends to degenerate (low shielding performance stability).
Japanese Patent Application No. 9-279422 deals with these problems by using a black resin layer containing dispersed black pigment instead of black dye. Although this ensures a high and stable degree of blackness and greatly improves clarity, it does not change the fact that the surface facing the display exhibits metallic glare and spoils viewability, i.e., overall clarity taking presence/absence of reflection into account is still not fully adequate.
On the other hand, ""682 is low in electromagnetic wave radiation shielding performance. For patterning in the chemical etching process, moreover, the only black layer usable when the electrically conductive material is copper (foil) is a black metallic layer soluble in the etching solution. Production of this black metallic layer is, however, very time-consuming and costly because it is formed by the wet chemical process widely used for copper (foil) roughening in the ordinary printed circuit board field (commonly called brown processing or black processing because the copper color is changed to brown or black by processing in an aqueous solution or the like of sodium chlorite, sodium hydroxide or trisodium phosphate).
Another problem is that the black metallic layer oxidizes the surface layer of the copper (foil). As this reduces the thickness of the formed metallic copper (foil) by the same amount, it degrades conductivity (electromagnetic wave radiation shielding performance).
In addition, the roughness of the black metallic layer surface makes its etchability extremely bad (formation of patterns with line interval under 200 xcexcm is difficult) and the surface roughness of the black metallic layer transfers to the adhesive layer at etched portions to make it non-transparent (like frosted glass). As use is impossible without restoring transparency, the adhesive layer is fused and press-bonded (with simultaneous lamination to the base material). This process is conducted at a high temperature of around 110xc2x0 C., limiting the type of base material to which it can be applied. (It cannot be practically applied to general-purpose acrylic plastic base materials or other such materials with low heat resistance because it causes them to warp markedly.) Further, the side facing the display exhibits metallic glare, so that clarity is poor owing to reflection. Still another shortcoming is that portions of the metallic layer not coated with the adhesive metallic layer readily oxidize in air. Conductivity (electromagnetic radiation shielding performance) therefore tends to degenerate.